Communications traffic at network edges is increasing over time due to the rising demand for a range of high-bandwidth services by business and residential customers. This rising demand places an increasing requirement on access networks to deliver those services.
One type of access network suitable for high-bandwidth services is a Passive Optical Network (PON). A PON typically has a central office (CO) at which apparatus called an Optical Line Terminal (OLT) interfaces with at least one metro or carrier network. An arrangement of optical fibres and splitters connect the Central Office to Optical Network Units (ONU) deployed across a service area. A Wavelength Division Multiplexed Passive Optical Network (WDM-PON) supports multiple wavelength channels, called lambdas. A separate wavelength channel is allocated for communication between an Optical Line Terminal (OLT) and an ONU in the WDM-PON.
It is desirable that apparatus deployed at ONUs of the access network is “colourless”. This means that the apparatus deployed at ONUs is not wavelength-specific but, instead, is capable of operating across a range of wavelengths. This allows an economy of scale in manufacturing the ONU apparatus. The configuration of the operating wavelength of an ONU is made by other apparatus in the network, external to the ONU.
There are several different approaches to WDM-PONs with colourless transceivers. One approach remotely seeds the ONUs. A seeding light is transmitted downstream from the OLT to ONUs. Each ONU receives a particular wavelength and a low-cost colourless source, such as a Fabry Perot laser diode, “locks” to the seeding light. The generated light is modulated with data to form an upstream optical signal. Another approach uses the same wavelength for downstream and upstream communication. An ONU receives the downstream signal, amplifies it, and modulates it with data to form an upstream optical signal. Both of these approaches require an external source at the OLT to generate the seeding light.
Another approach is called “self-seeding”. This approach is described in the paper “Directly Modulated Self-Seeding Reflective Semiconductor Optical Amplifiers as Colourless Transmitters in Wavelength Division Multiplexed Passive Optical Networks”, Wong et al, Journal of Lightwave Technology, Vol. 25, No. 1, January 2007. A Reflective Semiconductor Optical Amplifier (R-SOA) at an ONU generates broadband amplified spontaneous emission (ASE) light. The light is transmitted upstream. An Arrayed Waveguide Grating (AWG) at a remote node reflects a spectral slice of the broadband light back to the ONU for use as a seeding light. This locks the transmitted wavelength of the R-SOA. The R-SOA is directly modulated with upstream data. The approach described in the above paper has a tight power budget and only operates with a bit rate of 1.25 Gb/s, which is too low for some applications such as wireless backhaul.
The present invention seeks to provide an alternative self-seeded optical access network.